Freezing and melting of a sessile water droplet on a horizontal cold plate

Water droplet freezing phenomena widely exist in the fields of aerospace, power, meteorology and refrigeration. In this research, the freezing and melting processes of a sessile water droplet on a horizontal cold plate are studied experimentally. The nucleation temperature of a supercooled droplet is determined and analyzed statistically. The freezing behaviors of a water droplet, which are characterized by the freezing front and the droplet profile, volume and height, are obtained through the photographic approach. Experiments are done on water droplets having four different volumes including 1, 5, 10 and 20 μL. The experimental results show that the ice nucleation is stochastic, and its occurrence needs a large degree of supercooling. The nucleation temperature approximately satisfies the normal distribution, it scatters over a wide range, with the average nucleation temperature decreasing with reducing droplet volume. The droplet volume and height increase suddenly at the nucleation and recalescence stages, as time passes the volume expansion rate diminishes while the height increment rate increases. A colder plate yields a higher freezing rate and consequently a shorter freezing time. In contrast, a frozen droplet begins to melt at an almost fixed temperature of about 0.5 °C, and the larger the droplet, the longer the melting time.